Rhesus monkeys (M. mulatta) infected with the lentivirus, SIVmac, develop an immunodeficiency syndrome with marked similarities to HIV infection in humans. Approximately 70% of experimentally SIV-infected animals succumb to AIDS within one year, although the disease varies with virus strain, and the immune response of the host. We are using in situ hybridization with probes to SIV, SRV-1, CMV, EBV, Herpes saimiri, parvovirus, and SV40 to study tissue localization in primary retroviral- induced lesions and lesions induced by opportunistic infections in macaques experimentally infected with different strains of SIV.